This invention relates to apparatus and procedures for the location of faults in a variety of conduits or hollow conductors located above the ground which are used to protect wiring and cabling routed through them.
Conduit-type shielding for electrical conductors is frequently employed to protect the conductors against the high-current levels expected from nuclear electromagnetic pulse (EMP) effects and from lightning strokes. If faults in the conduits occur, then the electromagnetic shielding or hardening of a facility against the high current levels will be significantly reduced. Regardless of the fault location method, it is always necessary to locate the physical location of the fault by measurements taken in a variety of manners. Auxiliary general-purpose equipment is used to convert the parameters of a cable or conduit fault to certain defined states.
Prior patents have used a variety of techniques to determine fault location varying from inductive pulse techniques to using alternating voltages. For example:
U.S. Pat. No. 2,860,304, issued Nov. 11, 1958 to Hall, discloses a cable fault detector for determining fault location in electrical cables using an inductive pulse technique. U.S. Pat. No. 3,474,333, issued on Oct. 21, 1969 to Hoel, discloses a fault detector, using an alternating current system in which voltage drop is measured as a means for determining an inductance value for the system. U.S. Pat. No. 4,099,117, issued on July 4, 1978 to Erath, also discloses a fault detection technique using an alternating voltage. Electrodes are used to determine resistance values. U.S. Pat. No. 3,781,665 issued on Dec. 25, 1973 to Gale, discloses a fault detection system for coaxial cables. Alternating current, using an oscillator, is impressed across the system including the central conductor and the outer conduit. The fault is located as a function of the quarter-wavelength from the end of the cable to the fault. Input impedance is measured as a function of frequency to determine fault location. U.S. Pat. No. 3,792,350, issued on Feb. 12, 1974 to Bossler, is similar to the Gale patent previously mentioned and uses probes.
In general, the prior art detects a "major impedance change" or fault in a coaxial system by a reflection from that impedance change. A "major impedance change" means a short circuit between the center conductor and outer conductor or an open circuit in one of the conductors.
However, electrical currents large enough to disrupt sensitive electronic equipment operation or cause damage to such electronics can be coupled from current flowing on the external surface of conduits to wires inside the conduit if such conduits contain electrical discontinuities or defects even though such defects are not classified as "major impedance changes". Large external currents can result from lightning or electromagnetic pulse (EMP) effects. Apparently, no satisfactory technique exists for determining the quality and locating non-major defects of an electrical conduit system.